The present invention relates to using pressure in conjunction with micromachined beams for the capture of texture images, such as of a fingerprint, and methods of making and using the same.
The fingerprint sensing industry uses several different technologies to capture images of an individual""s fingerprints. The two most prominent technologies are optical based sensors and capacitance based sensors. Optical sensors use a light source, lenses and a prism to image the xe2x80x9cridgesxe2x80x9d and valleys on a fingerprint, based on differences in the reflected light from the features. The conventional capacitance sensor uses semiconductor CMOS processing to fabricate a two-dimensional array of capacitors on a silicon chip. The individual sensors on the chip form one plate of the parallel plate capacitor, while the finger itself, when placed on the array, acts as the second plate for the various localized sensors. Upon contact with the array of sensors, the individual distance from each sensor to the corresponding point on the skin above the sensor is measured using capacitive techniques. The difference in distance to skin at the ridges and valleys of a fingerprint provide the means to replicate the fingerprint. An example of the use of capacitive sensors to measure the spacing is shown in FIGS. 1A and 1B.
Both the above techniques, as well as other techniques used to capture fingerprint images fundamentally measure the spacing between the fingerprint features and the sensor. The measurement of spacing is inherently subject to several distortion effects: any parameter which affects the spacing between the finger and the sensor will affect the measurement. For example, both capacitive and optical sensors are very sensitive to oils or grease on the finger and to the presence or absence of moisture on the finger. In addition, the ambient temperature affects these sensors at the time of sensing. Under very hot or very cold conditions, the capacitive sensor can provide erroneous readings. Finally, most sensors have abrasion resistant coatings, and the thickness of the protective coating affects the measurement as it adds to the spacing. The combined effect of all these variables results in a very distorted image of the fingerprint, as shown in FIG. 1C. Finally, in the case of silicon chip based fingerprint sensors, the placement of the finger directly on to the silicon greatly increases the risk of electrostatic discharge and damage to the sensor.
As a result of the above drawbacks to spacing based reproduction of fingerprints, it would be very useful to be able to use the difference in pressure exerted by the ridges and valleys of a fingerprint at different locations on a sensor to replicate the fingerprint image. In principle, a pressure based fingerprint sensor would be impervious to the drawbacks listed above, such as wet or dry conditions on the fingertip, presence of oil or grease on the fingertip, thickness of protective coatings, etc, and would produce a xe2x80x9cpulsexe2x80x9d response, depending on whether each sensor in the array experiences a ridge or a valley. This situation is illustrated in FIGS. 1D and 1E, where the pressure sensor can highlight only the ridges, which are the lines of interest in a fingerprint. However, due to a variety of factors, pressure based sensors have not been deployed for the replication of fingerprints.
Accordingly, there remains a need for a device suitable for use as a texture image capture sensor that has high sensitivity, yet can provide high lateral resolution. Moreover, there further remains a need for a sensor that is suitable for use in fingerprint image capture that is less sensitive to adverse conditions such as extreme temperatures and skin oils and grease.
It is an object of the invention is to provide a pressure-based sensor that is suitable for use in texture identification and/or verification, such as for fingerprints and the like.
Another object of the invention is to provide a pressure-based sensor that is less sensitive to adverse conditions such as extreme temperatures and skin oils and grease.
Another object of the invention is to provide a pressure-based sensor that is less sensitive to transient ESD voltages and mechanical abrasion.
Another object of the invention is to provide a pressure based two dimensional array of sensors for use in texture capturing in which each element of the array can be electrically addressed and polled without the use of active semiconductor devices such as diodes or transistors.
The present invention fulfills these and other objects of the present invention by providing a pressure-based image capture device that includes an array of cantilevers or simply suspended bridges, with each pressure based sensor having a cantilever or a simply suspended bridge in contact with a conducting electrode that deforms under the load applied by the localized ridge of the texture, and which provides contact to another conducting electrode thereby closing the electrical circuit, a switch in the simplest form, and providing a xe2x80x9cpulsexe2x80x9d response from the sensor. In the quiescent state, each cantilever or simply suspended bridge structure contains an upper electrode which forms one part of the switch, while another conducting layer, the lower electrode at the bottom of the well of the individual sensor, forms the other part of the switch.
In the quiescent state, the cantilever or simply suspended bridge is electrically isolated from the bottom of the well, which is formed by the second electrode. When a texture is applied upon the array of sensors, some sensors fall under the ridges or protrusions on the texture, whereas the rest of the sensors are not in contact with any part of the texture such as between adjacent ridges on a fingerprint. Those sensors that are not in contact with any part of the texture continue to be electrically isolated from the lower electrode. However, those sensors that are directly under the ridges or protrusion of the texture deform under the load applied on the individual cantilever or simply suspended bridge, and with adequate deflection, the beam makes contact with the lower electrode, thus closing the electrical circuit.
In a preferred embodiment, each of the individual switches is connected in series with a very large resistance device, preferably a passive resistor, which is connected either to the row or to the column. Further, each of the columns is connected to a column multiplexer and each of the rows is connected to a downstream or row multiplexer. Further, a separate pull-down resistance device, again preferably a passive resistor, is connected from either every row or every column to a lower potential, such as ground, just prior to the connection to the multiplexer of interest. This allows the distinction between the ridges and valleys on a texture. For a fingerprint, all sensor locations where the electrical circuit is closed upon the application of a finger denote the ridges on the fingerprint.
Also, the present invention allows for the use of a membrane instead of a cantilever (also called xe2x80x9cbeamxe2x80x9d) or simply suspended bridge. In order to provide a generic descriptor no matter what specific structure is used to hold the upper electrode over a void disposed above the lower electrode; the term xe2x80x9csupportxe2x80x9d will be used.
While the above provides an overview of the invention, there exist numerous other significant aspects and advantages that will become apparent in the discussion provided hereinafter.